Screw threaded member



Dec. 25, 1962 R. c. BAUBLES 3,069,960

SCREW THREADED MEMBER Original Filed March 7, 1957 2 Sheets-Sheet INVEOR. RICHARD 6'. B BLES 7 Claim& (Ci. 35-1 This invention relates to anexternally threaded member, such as a screw, bolt, or other externallythreaded stud or article, and to method and apparatus for making thesame.

This application is a division of application Serial No. 644.548 filedby me on March 7, 1957, now abandoned.

The fatigue failure of externally threadzd members is of grave concernto many industries, particulary the aircraft industry, wherein repeatedloading of an externally threaded member commonly causes failure atstress levels which are far below those at which failure couldbe'expected under static conditions.

Fatigue failure of the threaded portion of an externally threaded memberhas heretofore always begun at the root of the thread.

It has been known for many years that a rolled thread is superior infatigue strength or fatigue life to a thread for'med by other methods,and it has previously been suggested that a thread formed by othermethods may be improved in respect to fatigue strength by finish rollingof the thread.

Externally threaded members formed by the thread rolling method aregenerally superior in fatigue strength to threads formed by othermethods and subsequently finish rolled in their crest or root portions.Moreover, finish rolling of the root portions of threads of the commonor most popular sizes used on aircraft is not feasible or practical fromthe standpoint of cost and satisfactory mass production tooling. It isaccordingly an object of this invention to provide an externallythreaded article having superior fatigue strength or lifecharacteristics and to devise a methzd and apparatus for making thesame.

Another' object of the invention is to provide a rolled thread capableof formation in blanks of greater hardness than have heretofore beendeemed capable of use for such purpose.

Other and further objects and advantages of the invention will beapparent from the following description When taken with the accompanyingdrawings in which- FIGURE 1 is a fragmentary view in elevation of anexternally threaded member embodying the invention;

FIGURE 2 is an enlarged, fragmentary view in section taken on the line2--2 of FIGURE 1;

FIGURE 3 is an enlarged fragmentary view in section through the axis ofa work piece showing a preferred form of die for making an articleembodying the invention, the parts being shown shortly after the die hasfirst engaged the work piece at the start of the rollirg operation;

FIGURE 4 is a view similar to FIGURE 3 showing the parts at the end ofthe rolling operation, and

FIGURE 5 is a View in section similar to FIGURE 2 but showing a furtherform of external thread.

Referring now to FIGURE l, the member or article 20, such as a screw,bolt, or other externally threaded stud or article, is provided with anexternal thread 22 having a root 24, a crest 26 and flanks 28.

The thread 22 in form and dimensions corresponds to a typical thread ofthe Unied, American or National thread form, with a thread angle of 60,but it is to be 33629360 Patented Dec. 25, 1962' 2 understood that theinvention is also applicable to external threads of other thread forms.

As shown in FIGURE 2, the root or root portion 24 of the thread 22 isrounded and the crest 26 fiat, the root 24 being a rolled or coinedsurface and the fiat 26 a machined surface. Each thread flank 28comprises a rolled or coined surface portion 39 extending from the rootto the pitch cylinder P and ground, or other machined, surface portion32 extending from the pitch cylinder to the fiat 26.

It should be understood that the thread 22 according to this inventionis formed on a cylindrical blank portion of a minimum tolerance diametersubstantially equal to the pitch diameter for the desired thread size.

In the manufaeture of high tensile bolts, the bolt blank is hardenedprior to thread rolling to a hardness above 37 Rockwell C and pre"erablybetween 42 Rcckwell C and 53 Rockwell C, or even higher.

At this point it may be stated that by the practice of this invention ithas been found possible to roll threads in material of higher hardness(and consequently higher tensile strength) than has heretofore beencapable of accomplishment.

The thread 22 is formed by a sequence of operations conssting of, first,a thread rolling operation to form a rol'ed or coined thread portionhaving the root surface 24 and the flank surfaces 30 inwardly of thepitch cylinder P and to form a freely extruded thread portion 34extending beyond the pitch cylinder P and having free extruded orextrusion surfaces. The thread rolling operation is followed by acutting or grinding step, or other surface machining Operations,simultaneously or sequentialy performed, to fcrm the flats 26 and theflank portions or surfaces 32 extending outwardly from the pitchcylinder P.

The thread rolling operation may be performed by the usual threadrolling mechanism employing, however, specially formed dies of which thedie 36, shown in FIG- URES 3 and 4, is illustrative. Each die 36 has afirst section including a crest portion 38 and flank portions 40extending inwardly from the crest portion 38 to the pitch line P' and asecond section including a root-portion 42 spaced from the crest portion38 a distance substantially greater than the depth of the thread to beformed by the die and joined by flank portions 44 to the flank portions49 at the pitch line P'. The included angle between the flank portions44 is substantially less than the included angle between the flankportions 49 so that an orifice through which the metal may be extrudedis formed at the junction of the flank portions 44 with the flankportions 40. The flank portions 44 and root portions 42 thus define arelief cavity or free expansion space 46 which beyond the extrusionorice, or inwardly of the pitch line P', is greater in Volume than theVolume of material which will be extruded thereinto by the coin ingoperation of the die upon the surface of the article or blank 20.

The flank portions or surfaces 44 of each die 36 are preferablyrelatively inclined at an included -angle of 29, each such flank surfacebeing formed preferably at an angle of /2 to the pitch line P', althoughit may vary with the composition or nature of the alloys or materialsbeing worked and with variations in size or form of the thread 22. Inany event, the angle of inclination of these flank portions 44 to thepitch line P' should be sufficiently great that the flank `surfaces 44do not come into work-shaping, pressure or coining engagement with thesurface of the material extruded into the cavity by the coiningoperation of the die upon the article 26.

During the rolling operation, a plurality of like dies s eeeo 36 aremoved in conventional fashion toward the axis of the article or body 20.Die crest portion 33 is the first part of each die 36 to engage the body2%), followed immediately by flank portions 40. These die portionstogetherexert a coining force on the body 20 to cause the surfacethereof to conforrn in every respect to such portions of the dies.FIGURE 3 shows the relationship of each-die 36 to the body 28 at anearly stage of the thread rolling operation.

i In continuing the thread rolling operation, the dies move material ofthe body 29 toward the pitch line P' of the die while continu ing tocoin the surface of the body and c'onfinirg material of 'the body 29, asin any coining operation, within the walls of the die. As the diescontinue to move 'inwardly toward the axis of the body 2%, the materialof the body, which has been displaced outwardly from the originalcylindrical surface thereof by the coining portions or surfaces 38 anddt) of the dies, f'ea'ehes the extrusion orifices at the pitch lines ofthe dies" and thereafter further inward movement of the dies toward theaxis of the body 20' causes the material displaced by the coiningoperation to be freely extruded through those orifices, into theextrusion cavities 46 of the'dies, and into shapes determined solely bythe extruion for'cesexertedbn the material by the coining operation ofthe dies `on` the metal body 20. Thus when the dies 36 have been movedthe required distance toward the axis of 'the body-20 so that the pitch'lines P' of the dies correspond, or substantially coincide, with thepitch cylinder `P',` i.e., substantially the original cylindricalsurface of the body 2%, as shown in FlGURE 4, the body 20 hasfrr'ned'therein and ther efrom a thread-like portion 47"'havingthecoine-d root portion or surface 24 for the final thread 22 and the fiankportions or surfaces 3% for the final thread 22 inwardly'of the pitchcylinder P and also 'having flank portions or surfaces 49, outwardly ofthe pitch cylinder P and recessed crests 51. It is to be ioted thattheflank portions 44 of the die cavities 46 do not exert any work-shapingpressures on the thread 47 or on the flank surface portions 49 of thatthread.

"During the described coining operation on the body 20, forces aregenerated within and along the surface of body 20 in a directionparallel to the surface of flank portions' 49 of the dies and away fromthe axis of the body 20. Forces are also generated within and near thesurface of' the body 20 in a 'direction perpendicular to and away fromthe axis of the body. The resultant effect of these forces is t'oproduce an extrusion force causing the material'as it is extruded intothe cavitics i-6 of the dies to expand beyond the normal thread formlines indicated by the dashed line N in FGURE 4 and to which thread fromthe material would be confined and shaped by'the walls of the prior artdie cavities. In orderto prevent the die portions or surfaces 4 4 fromapplying'substantial work-shaping pressures to the fianks ofthefreeextrusion portion of the thread 47 beyond the pitch cylinder P and pitchlines P' as aforesaid, it is necessary that such die flank portions beinclined at a greater angle' with respect to the pitch line P' than theflank surfaces of such free extrusion portion.

Following the forming of the thread-like form or portion 47 in and fromthe body 20, the fiank surfaces 4?? andthe crest surfaces 51 aremachined, as by a cutting, grinding 'or other surface machiningoperation, to the standard thread Outline, as in FIGURE 2, and asindicated by the dashed line N in FIGURE 4.

In FIGURE there is shown a thread 53 embodying the invention. Likethread 22, thread 53 may be made from thread 4-7. Thread 53 has a crest55 of the same major diameter as crest 26 of thread 22. The pitchcylinder of thread 53 is also indicated by line P-P. Thread 5 3 hasflank portions 57 further from the axis of thread 53 than pitch cylinderP-P and ank portions 59 Closer to the axis of the thread 53 than pitchcylinder P-P. Elank portions 57 make a smaller angle with the axis ofthread 53 than do fiank portions 59. The included angle between flankportions 57 is preferably on the order of 70. Thus outwardly of pitchcylinder P-P thread 53 is thinner than thread 22 at correspondingdistances from the thread axis. Thread 53 has the advantage in .that itmay be more readily formed from thread 47 by presently availablemachining tools than thread 22.

The hardened body 20 prior to threading is characterized by grainstructure flow lines indicating fiber orientation and extendingpredominantly parallel to the aXis of the body. On completion of thethread 22 'by the nethods herein disclosed, the flow lines near thesurface of the body 20 and throughout the thread 22 follow the pathsindicated by the lines 52 in FIGURE 2. These flow lines 52, indicativeof grain structure, follow generally the contour of the thread 22 andshow a highly compressed grain structure at the root of the thread; aparallel grain structure at and near the coined surfaces 36` andnonparallel grain structures intersecting the machined surfaces 32 at aslight angle and intersecting the machined surfaces of the fiat 26 at asharp angle, whereas in prior rolled threads the grain structureisuninterrupted. Cornparative fatigue tests of bolts having standardrolled threads and bolts provided with the external thread 22 of FGURE 2show a remarkable improvement in fatigue characteristics in favor of thethread 22. Such comparative fatigue tests showed, for example, that theaverage cycles to failure of a /8-24 thread of a k.s.i. bolt tested at astress level of 120,(3-30 p.s.. had been increased, on the average `over3 times, and When tested at a stress level of 83,000 p.s.i. hadbeenincreased on the average, over twenty' times and that the average cyclesto failure of a 220 k.s. i. bolt was increased, on the average, fromthirty to two hundred times :for a wide range of Stress levels.

For highest fatigue strength or life, it is essential that at and nearthe root surface of an external thread, where the tensile Stress underapplied load will be greatest, a residual Stress be induced which iscompressive'rather than tensile. The higher the residual compressivestresses at and near the root surface, the greater will be the fatiguestrength or life because the greater the residual compressive stressesat and near the root 'surface the less the probability of a tensilefatigue crack. Hence, the externally threaded article will be able towithstand for a longer time or for more cycles of loading thefailurecausing tensile stresses exerted generally `axially of thethreaded article. r

The present invention achieves a substantial increase in the residualcompressive Stress of the thread root area. The theoretcal reason -forthis increase and the resultant improvement in fatigue characteristicsappears from' the following analysis.

In the case of a thread rolled in accordance with the hitherto acceptedstandard practice, all 'surfaces of the external thread including thefiank and crest surfaces outwardly of the pitch cylinder, or theexternalcylin drical surface of the blank, are coined by the formingpressure of the die crest surfaces 'and the surfaces of the diecavities, including the die cavity surfaces inward of the pitch lines ofthe dies. The coiningof the fiankand root surfaces of the threadinwardly of the pitch cylinder tends to create desirable compressivestresses at the root 24 while the coining of the surfaces outwardly ofthe pitch cylinder if performed according to standard practice tends toreduce the magnitude of the residual compressive stresses at the root.

Since in the formation of the thread 22 the thread portion 34 outwardlyof the pitch cylinder is freely extruded and the fiank and crestsurfaces of that extruded portion of the thread are not formed byacoining operation, but by cutting, grinding, or other surface machiningOperations, compressive or back pressure stresses-are not created'in theextruded portions 34 and, therefore, the advantageousresidualcompressive stresses created &069,960

by the coining of the ank and root surfaces inwardly of the pitchcylinder are not dininished and remain at the maximum value in thefinished part.

In View of the well known fact that the heat treatment of a threadedmember removes previously induced residual stresses, and may induce newlargely uncontrollable, residual stresses, it is essential to thesecurement of the benefits of this invention that the member, body orarticle to be threaded be hardened prior to the thread formingoperation.

The forming of the external thread 22 according to the methods hereindisclosed causes a substantial increase in the hardness of the coinedsurfaces 24 and 30 of the member or article, as for example, from ablank hardness of 42 Rockwell C to about 50 Rockwell C, while thehardness of the remaining surfaces, as for example the crest surface, issubstantially unaffected.

Measurements made, by the standard three-wire method of measuring pitchdiameter in order to determine changes in the pitch diameter resultingfrom release of the residual stresses at the root of the thread for astandard rolled thread and for the thread 2?. have established that theresidual compressive stresses at the root of the thread 22 areremarkably greater than any residual compressive Stress at the root ofany standard thread.

For some usages, the final machining of the fiank surfaces outward ofthe pitch cylinder, and/or the final machining of the crests, may beeliminated.

This invention also enables threads to be formed on harder blanks thanhas heretofore been possible. Thus, with methods of the prior art it maybe possible to roll threads on blanks of a maximum hardness of about 47Rockwell C, whereas by this invention threads can be rolled on blanks ofa hardness up to 52 or 53 Rockwell C, and even higher. This means thatparts of considerably higher ultimate tensile strength can be producedby the practice of this invention than has heretofore been possible.

While there have been illustrated and described herein the preferredembodiments of an external thread embodying the invention, and ofn'ethods and apparatus for making such thread, it will be appreciatedthat this has been done to enable those skilled in the art to appreciateand practice the invention, the true scope of which is indicated by theappended claims.

What is claimed is:

1. The method of forming an external thread in a metal body, comprisingthe steps of coining the surface of the body into the form of the finalroot surface and the final flank surfaces of the desired thread inwardlyof the pitch cylinder of such thread and simultaneously andsubstantially freely extruding the material of said body 'outwardly fromwithin the pitch cylinder through an orifice to form the sections of thedesired thread beyond the pitch cylinder without substantial work-shapngpressures.

2. An externally threaded article of which the external thread comprisesa coined root, and coined flank surfaces inwardly of the pitch cylinderand a portion simultaneously and substantially freely extruded throughan orifice and having machined surfaces extending outwardly from thepitch cylinder.

3. An externally threaded article of which the external thread comprisesa coined root and coined flank surfaces inwardly of the pitch cylinderand a portion simultaneously and substantially freely extruded throughan orifice and having machined surfaces extending outwardly from thepitch cylinder, said thread having a predetermned 6 thread angleinwardly of said pitch cylinder and a predetermined greater thread angleoutwardly of said pitch cylinder.

4. An externally threaded article of which the external thread includesrolled sections inwardly of the pitch cylinder of the thread andsections simultaneously and substantially freely extruded through anorifice and extending outwardly from the pitch cylinder.

5. A work hardened metal stud formed from a metal blank prehardened to apredetermined hardness and having an external thread comprising coinedsections inwardly of the pitch cylinder and sections simultaneously andsubstantially freely extruded through an orifice and extending outwardlyfrom the pitch cylinder, the root of the thread being substantiallyharder than the crest of the thread.

6. The method of forming a high fatigue life, ex` ternally threadedmetal body comprising the steps of rolling a helical groove in acylindrical prehardened body 'of a minimum external diametersubstantially equal to the pitch diameter of the desired thread untilthe bottom of the groove is of the desired root diameter and adjacentflank surfaces of the groove at the desired thread angle whilesimultaneously and substantially freely outwardly extruding from thebody between adjacent flank surfaces and through an orifice outer threadportions having extrusion crest surfaces and extrusion flank surfacesextendng from the rolled fiank surfaces of the groove at an includedangle substantially less than the desired thread angle and for adistance from the bottom of the groove greater than the desired threadheight, and thereafter surface machining the extrusion crest surfacesand the extrusion flank surfaces to the desired thread crest diameterand thread angle.

7. A work hardened metal stud formed from a prehardened metal blank andhaving an external thread comprising a rolled helical groove within `animaginary cylinder of a diameter substantially equal to the pitchdiameter of the desired thread and sections outside said cylindersimultaneously extruded through an orifice and shaped by surfacemachining operations to the desired crest diameter and thread angle.

References Citei in the file of this patent UNITED STATES PATENTSl,865,575 Locke July 5, 1932 2,011,761 Handel Aug. 20, 1935 2,l50,876Caminez Mar. 14, 1939 2,335,590 Gersman Nov. 30, 1943 2,364,442 HoernDec. 5, 1944 2,514,589 Penman July 11, 1950 2,656,740 Bedker Oct. 27,1953 2,679,774 MacDonald June 1, 1954 2,690,089 Bedker Sept. 28, 19542,699,077 Bedker Jan. 11, 1955 2,750,034 Gersman June 12, 1956 FOREIGNPATENTS 564,269 Great Britain Sept. 20, 1944 751,103 Germany Sept. 22,1952 OTHER REFERENCES An investigaton of the Laws of Plastic Flow,Scientific Paper No. 278 by Eugene C. Bingham, Bureau of StandardsBulletin, volume 13, 1917, QC 1 US.

The Extrusion of Metals, Pearson, chapter 5, pages 98-113, published byJohn Wiley and Sons Inc., 1953.

American Machinist, page 154, July 9, 1951.

